More than half of next year's expected 5.7% increase in global IT spending will be driven by sales of smartphones, tablets and e-readers, IDC said in a forecast published Friday.

IDC projects that mobile device sales will make up "an incredible" 57% of overall IT spending growth expected in 2013.

Without the mobile spending, the IT business would grow by 2.9%, about the rate of GDP growth worldwide, IDC noted.

Mobile devices were also the biggest segment of IT spending growth in 2012, IDC said.

Sales of mobile devices will total $431 billion in 2013, an increase of 20% over 2012. Mobile devices will account for 20% of all IT spending in 2013, which IDC puts at $2.1 trillion worldwide.

Some mobile device sales will come at the expense of PCs.

IDC predicts that 2015 will mar the first time more U.S. consumers access the Internet through mobile devices than from PCs.

By 2016, IDC predicts, there will 225 million PC users, and 265 million users of mobile devices.

The report, "IDC Predictions 2013: Competing on the 3rd Platform," describes an evolution of IT toward a third platform built on mobile devices, cloud services, social technologies and Big Data.

Those were important IT spending areas in 2012, but IT will see "much greater urgency in 2013, and much bigger moves, as the market shifts beyond the exploration stage to the full-blown high-stakes completion," IDC said in the report.

IDC defines the first platform as the mainframe computer, following by the second platform of the PC, which greatly impacted computing from 1985 to 2005.

Regarding the third platform, mobile devices, IDC said 2013 will see a surge in sales of mini-tablets with sub-8-in. screens. The "minis" will make up 60% of all tablet shipments in 2013, a "remarkable leap from just 33% in 2012," IDC said.

The influence of mini tablets and their prices in the $200 to $400 range will start a trend by school districts to replace textbooks with tablets, IDC also predicted.

"Look for a viral expansion of tablet pilots in school systems in 2013, accelerated by the explosion in online learning resources (such as massive open online courses) and the decline of the paper-based textbook publishing industry," the IDC report said.

The smaller, cheaper tablet will proliferate into other areas as well, like health care, said IDC analyst Frank Gens, author of the report, in an interview.

IDC also warned that Microsoft and Research in Motion need to attract more developers and apps to their Windows and BlackBerry mobile platforms in 2013 to avoid a "gradual track to demise for their platforms."

"Microsoft's got a big climb ahead of it. It's fighting like crazy with the Surface tablets and the Nokia partnership, but the clock is ticking very loudly," Gens said.

"As we point out, 2013 will be where [Microsoft and RIM] make the leap, or fade out, in the mobile space," Gens added.